Its definitely a Tapajos red. Can you get some clear shots of the dorsal? Usually females of this species will have a white stripe on their dorsal and tail fins. Since I don't see a white stripe, I would say this guy is male.
There is no white stripe outside of edging on the tail fin, I'm wondering at what age/size do the stripes show up? How large can I expect a male to grow?
It is still a little early to be telling. Generally the stripe doesn't come into effect until they are 8 to 10", sometimes it takes them even a little longer than that. Just be patient and wait it out. Having said that, at the moment it does appear to be male.
The great thing about sp. 'tapajos I' is that male and female look practically alike. The only thing that really distinguishes them is the white stripe in the dorsal on females. Both genders get the rich, vibrate red coloration.